In the cultivation of plants, e.g. vegetables, with balled seedlings, the aim is to obtain a complete stand of plants in the field. For different reasons, after the germination stage the seedling trays usually contain a number of balls without seedlings. These empty balls were earlier sorted out manually before mechanical transplanting, or alternatively they were replaced with seedling-carrying balls following transplanting. In practice, the consequence of this is that the planting process is semiautomatic, whereby an operator supplies the machine only with seedling-carrying balls. Alternatively, automatic transplanting is first used, whereby the empty balls are set in the ground and a person walking behind the machine manually replaces the empty balls with seedling-carrying ones. This eliminates part of the economical benefit gained by the use of a continuously operating transplanting machine. If sorting or replacement is not carried out, the result is patchy field and crop losses.
To overcome the abovementioned disadvantage, it has been attempted to, e.g. remove the empty balls and fill the opening by, for example, temporarily speeding up the rate by which seedlings are fed to the planting chute. Characteristic for these methods are the difficulty of detection, use of complicated mechanical structures and vigorous handling of the ball and seedling.
From International Patent Application WO 91/10352, a detection method for plants, based on fluorescence, is known. According to said method, the fluorescence characteristic for, e.g., the chlorophyll is a plant, is detected by means of light filters and a camera. The method, however, requires relatively complicated optics and data processing.
In Japanese patent application 8-37832, a transplanting device is disclosed, comprising a mechanical switch for the detection of empty seedling balls. Detection requires physical contact with the stem or leaves of the seedling, and takes place on seedlings still within the cells of the cultivating tray.
Japanese patent application 8-112012 discloses a transplanting device where two similar cultivating trays are used as sources of seedlings, one as the main source and the other as a replacement source. Detection of the seedlings takes place on seedlings still within the cells of the tray, as in the previously mentioned Japanese application, and when a deficient specimen is detected, a seedling is extracted from the replacement tray instead. Seedlings from either tray are transferred into the planting chute through a Y-shaped tube.
Japanese patent application 8-103112 discloses a transplanting device where individual balled seedlings are carried from the cultivating tray to cups in a carousel-type transfer device, where detection takes place. The seedling is detected above the rim of the transfer cups. A shutter mechanism opens to drop an accepted seedling into a planting device, while rejected seedlings are carried off and substituted by replacement seedlings through a side channel opening into the planting chute. The side channel is supplied with individual replacement seedlings during use.